Fix `nil.empty?` error

Fixes error when creating a new bucket object. (Issue #5).

Some additional housekeeping:

## Adding `Rakefile`

Adds `Rakefile` to support Bundler and Rake::TestTask tasks

Run `bundle exec rake -T` to see available tasks.

```bash
beaucollins@Collins simperium-ruby (tests)$ bundle exec rake -T
rake build    # Build simperium-0.0.2.4.gem into the pkg directory
rake install  # Build and install simperium-0.0.2.4.gem into system gems
rake release  # Create tag v0.0.2.4 and build and push simperium-0.0.2.4.gem to Rubygems
rake test     # Run tests
```

## Add Development Dependencies

Development dependencies are now defined in `simperium.gemspec` so a `bundle install` will get you everything you need to get rolling with development.